Add format function for scaled values

Returns an integer when evenly divisible by scale, otherwise
a string using the given number of decimal places (default 1)
Add rulings, returnable_card_fields

rcf constrain which values can be returned, and in the case of fields
with multiple representations (power, toughness) how they are
returned.  Note that the keys of this dictionary do not necessarily
correspond to the values that a consumer would pass in.  For instance,
'rules' would likely be passed as 'o', 'type' as 't'.
Add controller filter function for card fields

This function takes a card and returns a dictionary only containing
the requested fields.  Keys in the 'fields' dictionary should be a
strict subset of the keys in models.returnable_card_fields.  Any key
that is not in returnable_card_fields is ignored.